1. Field of the Invention
The present invention relates to a radiological image pickup apparatus used in a medical, industrial, or nuclear field, etc., for amplifying an electric signal taken out from a radiation detector, which detects the spatial distribution of incident radiation, by an electric signal processing circuit and creating a radiological image by an image processing circuit based on the electric signal amplified by the electric signal processing circuit. The present invention relates in particular to an art for making it possible to circumvent fluctuations in detection sensitivity in the radiation detector for detecting the spatial distribution of incident radiation and further occurrence of residual output without narrowing the dynamic range of the electric signal processing circuit.
2. Description of the Related Art
A radiological image pickup apparatus in a related art includes a flat-panel radiation detector of direct conversion type, an electric signal processing circuit, and an image processing circuit. In the flat-panel radiation detector of direct conversion type, a common electrode for applying a bias voltage is formed on one side of a semiconductor layer sensitive to radiation (for example, X-rays) and a plurality of split electrodes are formed on an opposite side. Charges occurring in the semiconductor layer as radiation is incident are taken out as an electric signal from each of the split electrodes so that the spatial distribution of the incident radiation can be detected. The electric signal processing circuit amplifies the taken-out electric signal. The image processing circuit creates a radiological image based on the electric signal amplified by the electric signal processing circuit. Such a radiological image pickup apparatus in the related art involves problems of fluctuations in detection sensitivity of the radiation detector and occurrence of residual output in the radiation detector.
That is, the flat-panel radiation detector of direct conversion type in the related art does not have electrodes at the space between the split electrodes, from which charges moved by an electric field to the space between the split electrodes are taken out. Further, the flat-panel radiation detector of direct conversion type in the related-art has the property that charges occurring as radiation is incident is easy to be accumulated at the space between the split electrodes. Consequently, as space charges gradually accumulate in the space between the split electrodes while radiation is incident, distortion of the electric field gradually develops and the effective sensitive area changes, and therefore a phenomenon of fluctuations in the detection sensitivity occurs. In this case, after incidence of radiation stops, the space charges accumulating in the space between the split electrodes are gradually taken out from each split electrode, and therefore a phenomenon of occurrence of residual output also occurs.
Another type of a radiological image pickup apparatus in a related art includes a flat-panel radiation detector of indirect conversion type, an electric signal processing circuit, and an image processing circuit. In the flat-panel radiation detector of indirect conversion type, a plurality of semiconductor optical sensors provided by electrically splitting a semiconductor layer sensitive to light are formed on one side of a radiation-light conversion layer for converting radiation (for example, X-rays) into light. Light occurring in the radiation-light conversion layer as radiation is incident is taken out as an electric signal from each of the semiconductor optical sensors so that the spatial distribution of the incident radiation can be detected. The electric signal processing circuit amplifies the taken-out electric signal. The image processing circuit creates a radiological image based on the electric signal amplified by the electric signal processing circuit. Such a radiological image pickup apparatus in a related art also involves problems of fluctuations in detection sensitivity of the radiation detector and occurrence of residual output in the radiation detector.